(Single-Photon-Counting System of Radiation)
A single-photon-counting detector of radiation includes a number of pixels and performs counting inside thereof. Such a detector is used by reading the result of counting for exposure for a fixed period of time. In particular, it can be thought to apply a detector supposed to have adaptation to highly intense radiation at a high counting rate, in addition to the minute pixel size, to high-speed imaging, specimen observation in a brief time, an analysis method, etc.
The detector as described above has imaging cells sensitive to radiation. Then, the signal from a read cell connected to the imaging cell is digitized by the single-photon-counting system. The count number that is digitized is further read by a reading device placed outside.
The signal (charge) from the imaging cell becomes a digital signal in accordance with the level of radiation energy by a wave height discriminator circuit through a charge amplifier circuit and a waveform shaping amplifier circuit. Then, the digital signal is counted by a counter circuit including a shift register. In the detector, together with the imaging cells, read cells including these circuits are arranged in the form of a matrix.
(Performance of High-Speed Measurement)
When an attempt is made to implement high-speed measurement with such a detector, it is necessary to omit wasteful time and to record incident photons by a counter circuit. In this case, the outputs from the charge amplifier circuit and the waveform shaping amplifier circuit are input to the wave height discriminator circuit as pulses. However, the temporal width of this pulse, that is, the response time of these circuits is wasteful. It is possible to implement high-speed performance by shortening the response time.
(Offset Voltage)
On the other hand, in the circuit with the configuration described above, it is known that there exists an offset voltage to the input signal in each of the charge amplifier circuit, the waveform shaping amplifier circuit, and the wave height discriminator circuit. That is, even when the input becomes zero, the output does not become zero. Further, a bias voltage is applied when the imaging cell sensitive to X-rays is used, however, there exists a leaked current (DC current) in the output. If the leaked current is input to the charge amplifier circuit as it is, the amplifier circuit itself is biased by the current as a result, forming another cause to generate the above-described offset voltage. The values of these offset voltages differ in magnitude between different imaging cells, and therefore, if an imaging device is configured in this state, the position where the signal appears, which corresponds to the incident photon of the X-ray input, differs from one another for each read cell and it is not possible to correctly form an image.
(AC Coupling)
As one method to avoid this, a capacitor is inserted in the previous stage of the charge amplifier circuit and at the input and output of the waveform shaping amplifier circuit to cut the DC component of the leaked current. FIG. 5 is a diagram showing a circuit configuration of a radiation detector 500 by the conventional AC coupling. The circuit system as shown in FIG. 5 is called the AC coupling.
As shown in FIG. 5, the radiation detector 500 includes a bias potential supply source 501, a threshold voltage supply source 502, an imaging cell 504, a read cell 507, a feeding unit 513 configured to read a counter circuit, and a signal line 514 connected from the counter circuit to outside. The imaging cell 504 includes a photodiode 503, an output terminal 505 from the photodiode, and a connection point 506 for the bias potential. To the photodiode 503, a reverse bias voltage is applied by the bias potential supply source 501 and a current signal is generated in response to the incidence of radiation. The read cell 507 includes an amplifier circuit 508, a capacitor 531, a wave height discriminator circuit 509, a counter circuit 510, an input terminal 530, and the signal line 514. FIG. 5 shows only a circuit corresponding to one of the plurality of imaging cells (this also applies to the following drawings).
The pulse signal having passed through the capacitor 531 by the AC coupling tends to overshoot at the time of rise and undershoot at the time of fall. In the AC coupling, the next pulse signal cannot be received while the pulse is passing therethrough. Further, even in the state where undershoot has occurred, if the next pulse signal overlaps, the wave height value changes, and therefore, it is not preferable to receive the pulse signal. That is, in the AC coupling, it is not possible to narrow the interval between pulse signals beyond a certain limit. That is, the high-speed pulse signal processing becomes difficult, and therefore, in order to implement high-speed measurement as a challenge, it is not preferable to adopt the AC coupling.
(DC Coupling)
On the other hand, a configuration in which a capacity is not inserted in each stage and each stage is connected directly is called the DC coupling. FIG. 6 is a diagram showing a circuit configuration of a radiation detector 600 by the conventional DC coupling. For the above-described reason, when high-speed performance is aimed at, it is preferable to adopt the DC coupling for connection, to place a DA converter 612 that can be set individually from outside within each read cell, and to make an attempt to cancel out the offset value by performing analog addition of the output thereof and the output of the waveform shaping amplifier circuit (amplifier 608 in the subsequent stage).
As shown in FIG. 6, the radiation detector 600 includes a bias potential supply source 601, a threshold voltage supply source 602, an imaging cell 604, a read cell 607, a setting unit 611 to a DA converter, a feeding unit 613 configured to read a counter circuit, and a signal line 614 connected from the counter circuit to outside. The imaging cell 604 includes a photodiode 603, an output terminal 605 from the photodiode, and a connection point 606 for the bias potential. To the photodiode 603, a reverse bias voltage is applied by the bias potential supply source 601 and a current signal is generated in response to the incidence of radiation. The read cell 607 includes the amplifier circuit 608, a DA converter 612, a wave height discriminator circuit 609, a counter circuit 610, an input terminal 630, and the signal line 614.
Patent Document 1 describes an example of a circuit of the DC coupling as described above. As shown in FIG. 4 of Patent Document 1, in order to implement adaptation to a high counting rate, the output from CS AMP is connected directly to CA COMP inside of ANALOG BLOCK. The output of CS AMP has an offset value having a value different for each different read unit cell and in order to cancel out this, THRESHOLD CORRECTOR is attached to the other input of CA COMP.    Patent Document 1: U.S. Pat. No. 7,514,688